1. Field of the Invention
The invention relates to memory systems and more particularly to memory systems in which dot matrix pictorial information is stored and accessed periodically to supply data to a raster device.
In graphical display systems, graphic orders which are encoded in the form of lines drawn between XY coordinate pairs are converted into a visual image and displayed on a cathode ray tube type of display. The graphical data is presented in a matrix of dots that are modulated onto the video raster of the cathode ray tube. The present invention is concerned with the type of system in which each dot on the display is refreshed by a refresh memory with one memory bit corresponding to each dot on the display. Hence, in prior display systems of this type, a raster containing 1,024 .times. 1,024 dots required a memory to hold the picture information which would be approximately one million bits in size. This type of memory is referred to as a full bit map memory, since there is one bit for each dot on the display screen.
The encoded graphical data to be displayed on the screen must be converted into points which are plotted into the full bit map memory. Since the graphical data consists of line segments which may start and end at any point on the screen, it is necessary that the full bit map memory be a random access memory so that points are plotted at the maximum possible data rate.
A major drawback of the full bit map memory approach is the high cost of the refresh memory. It is therefore commercially advantageous to reduce the cost of the full bit memory.
Since there are large open spaces in line-drawn images, the open spaces can be compressed by using fewer bits per picture element by data compression techniques. In the case of refresh memories in graphic systems, the technique chosen must be such that the compressed data is easily changeable so that points may be added or deleted from the screen without having to regenerate the whole image each time a change is desired.
It is therefore a paramount object of this invention to provide a compressed refresh memory which is smaller and less expensive than a bit-per-picture element memory for the equivalent function. The only exception to this would be when the compression factor for the compressed memory is exceeded. However, one can always guarantee to attain a certain compression factor by magnifying the image to be presented on the screen. A magnification by a factor of two, for example, multiplies the open space area by four, thus greatly increasing the possible compression factor.
It is a further object of this invention to enable points to be added or deleted from the compressed memory representation of the image without having to rewrite the entire memory contents.
A further object of the invention is to provide a random access memory facility with the compressed memory so that point plotting and deleting is done at a speed which is substantially higher than a serial access memory.
It is an object of this invention to provide a compressed refresh memory which has the same functional characteristics as a full bit memory.
It is another object of this invention to provide a compressed refresh memory which can be accessed in a row major format to supply data to a raster device.